The present invention relates to a flight control system for a hybrid aircraft, and more particularly, to a flight control system for a hybrid unmanned aerial vehicle (UAV) which blends command signals to a multiple of vehicle control surfaces during transition between rotor borne and wing borne flight.
There is an increased emphasis on the use of UAVs for performing various activities in both civilian and military situations where the use of manned flight vehicles may not be appropriate. Such missions include surveillance, reconnaissance, target acquisition, target designation, data acquisition, communications relay, decoy, jamming, harassment, ordinance delivery, or supply.
A hybrid aircraft provides the hover and low-speed maneuverability of a helicopter with the high-speed forward flight and duration capabilities of a winged aircraft. Typically, hybrid aircraft include a helicopter control surface system which provides cyclic pitch, collective pitch and differential rotation to generate lift, pitch, roll, and roll control when operating in a hover/low-speed environment. Additionally, the hybrid aircraft includes a conventional fixed wing aircraft control surface system such as aileron, elevator, rudder and flaps to provide control when operating in a high-speed environment. Hybrid aircraft also typically include a separate translational propulsive system.
When the hybrid aircraft is operating in a hover/low-speed environment, maneuverability is achieved by controlling the helicopter control system. When the hybrid aircraft is operating in a high-speed environment, the hybrid aircraft operates as a fixed wing aircraft and maneuverability is achieved by controlling the aircraft flight control surfaces. As the hybrid aircraft transitions between helicopter and aircraft control surface systems, however, neither the helicopter nor the aircraft control systems are completely effective. Moreover, the relationship between control displacement and control moment is nonlinear and the aerodynamic forces on the aircraft change most dramatically. Flight control within this region is therefore rather complex.
Accordingly, it is desirable to provide a hybrid aircraft flight control systems which automatically blends command signals to a multiple of vehicle flight control surfaces during transition between rotor borne and wing borne flight. It is further desirable to efficiently control vehicle pitch during transition so that sufficient lift and control throughout the transition is maintained.
A hybrid aircraft according to the present invention can hover like a helicopter using a rotor system or fly like a fixed wing aircraft using conventional fixed wing controls such that it is operable in four flight regimes:
1. Hoverxe2x80x94Defined as low speed operation. The rotor generates control and lift.
2. Forward Flightxe2x80x94Lift is generated by the wings and all control is through the fixed wing surfaces (elevator, ailerons, rudder)
3. Transition Upxe2x80x94This mode guides operation of a multiple of control surfaces when flying from Hover to Forward Flight.
4. Transition Downxe2x80x94This mode guides operation of a multiple of control surfaces when flying from Forward Flight to Hover.
The flight control system according to the present invention includes a lift control algorithm which selectively communicates with the pitch command of the flight system control algorithm. The lift control algorithm controls the pitch attitude of the vehicle when the collective control is enabled and when the vehicle is in transition up/down mode. That is, the lift control algorithm control vehicle pitch during transition between hover and forward flight.
The initial power-up flight mode of the vehicle is Hover. If the vehicle is commanded to fly faster than the transition Up threshold, transition Up mode is entered. During transition Up mode, a transition logic circuit compares the collective command with a low thrust threshold. As vehicle speed increases, the wings create more lift and the flight control law strategy decreases the collective pitch command to maintain a desired vertical control. When the collective pitch command reaches the low thrust threshold, the rotors are maintained at flat collective pitch and are fixed in cyclic pitch. The flight mode is then changed to forward flight mode.
Forward flight mode is maintained so long as the vehicle speed at which the aircraft is commanded to fly exceeds the Transition Down Threshold. It should be understood that the Transition Down threshold is not necessarily the same as the Transition Up threshold. As vehicle speed decreases, the wings generate less lift and the flight control law strategy must increase their angle of attack to maintain desired vertical control. During the Transition down mode, the transition logic circuit compares the commanded pitch attitude reference with an angle of attack schedule. When the commanded pitch attitude reference reaches an angle of attack threshold the collective command is again directed to the collective control such that other than flat pitch is available.
During the transition modes, the lift control algorithm controls the pitch attitude. The lift control algorithm selects the proper control commands to coordinate an effective transition between hover and forward flight. The most efficient vehicle pitch during transition is thereby automatically generated so that sufficient lift and control throughout the transition is maintained. The present invention therefore provides a hybrid aircraft flight control system which automatically control transition between rotor borne and wing borne flight.